Copyright 1991, Rick Chinn. All rights reserved.
This article was originally published in Audio/Video Interiors
体彩七位数18178<b><font size="+4">Speakers of the House
If you've only got two pairs of speakers in your home audio system, then
the A-B switch on your receiver is probably more than adequate. But what
if you've got 10 pairs? What about impedance matching? What about remote
As you'll see, you can't just connect the 10 pairs of speakers up and have
things work. They may, for a while, but sooner or later, something will go
wrong (the resident teenager has a party and tries to listen at concert
volume) and you'll need to send your amplifier or receiver to the hospital.
Even if you manage to make it work, there's the problem of volume control.
Running back to system central to make an adjustment will get old quickly,
even if you've got help from an accomplice at the remote location. The
obvious thing is to install some sort of volume control at the other end.
In days past, you went to the electronics store and bought an "L pad."
Unfortunately, high-wattage L pads are difficult to find, and lower wattage
units tend to burn out. Worse, they turn amplifier power into heat that
you paid good money for, a function better reserved for your furnace.
There are several common systems for distributing audio throughout a home
or other building. We'll touch on four of the more common ones here:
parallel distribution, multiple amplifier, constant voltage (70-volt), and
low level. Wired remote systems, such as those made by Niles Audio and
Audio Access are beyond the scope of this article. Suffice it to say that
these systems use a combination of parallel distribution and multiple
amplifiers. If you don't understand these terms yet, read on...by the end
of the article, you will.
The most basic audio distribution system consists of two speakers connected
to an amplifier. From here on, we'll only consider one of the stereo
channels...the other channel is a duplicate of the first. In this system,
the amplifier must be able to drive the parallel combination of the two
speakers. If each speaker is 8 ohms, then the amplifier sees 4 ohms. Most
amplifiers can drive a 4 ohm load without difficulty...but what if we
decide to expand the system? Let's try four speakers on one amplifier.
Four 8-ohm speakers connected in parallel present a combined load of 2 ohms
(calculate by dividing the number of speakers by the quantity) to the
amplifier. With rare exception, most home amplifiers will not tolerate
this much of a load without distress (read smoke). Every amplifier made
has voltage and current limits at its output. Reaching the voltage limit
is also known as clipping. If this were a perfect world, amplifiers would
not have current limits. This would allow connecting an infinite number of
speakers. As the load impedance drops, the amplifier's power output rises.
For example, an amplifier rated at 100 watts/8-ohms would deliver 400 watts
into 2 ohms. Into a 1/2-ohm load, the same amplifier would deliver 1600
watts! Sadly, we live in an imperfect world, and amplifiers have current
limits, which determines the minimum load impedance that may be connected
to the output terminals.
By now, you can probably see the need for a better way. Indeed, there is.
Previously, I mentioned four basic systems: parallel distribution,
multiple amplifiers, constant voltage, and low level. Before getting into
the fine details of each, let's take a brief look at each one.
Parallel distribution systems are an extension of the A-B switch found on
most receivers. Quite simply, every speaker in the system is connected in
parallel and thence to the amplifier. As pointed out earlier, the major
disadvantages are: the rather abnormal load presented to the power
amplifier (there are ways of dealing with this), the wire size required to
keep wire loses to a minimum, and volume changes as various zones/rooms are
switched on and off. A major advantage is simplicity.
Multiple amplifier systems get around the abnormal load impedance problem
very simply (and somewhat elegantly): give each load its own amplifier.
This could take the form of a stack of integrated amplifiers located at
system central, or a card cage (rack) of amplifier boards located in the
garage. This is an effective solution with only a few drawbacks: the cost
of the multiple amplifiers, the space required, wire cost, and somewhat
higher system complexity. The major advantages are: the lack of
interaction between different rooms or zones, and the ability to equalize
each zone separately. In this case, equalization helps smooth out the
overall frequency response curve. Since the acoustics in each zone are
probably different, it makes sense that each zone will require separate
equalization. In all fairness, the cost of the multiple amplifiers may be
only slightly higher than the price of one large amplifier with sufficient
power to drive the entire house.
Constant voltage systems operate much like the power company does in your
home. At home, you plug appliances into a single circuit, until the
circuit breaker or fuse blows, then you back up one appliance, right? In a
constant voltage system, you think of each load in terms of the number of
watts it consumes. The total of all loads (speakers) must not exceed the
amplifier power. The disadvantages: a slight increase in complexity, a
certain amount of thought is required, some golden-eared types might object
to the presence of transformers in the signal path, and the extra cost of
the transformers (one required per speaker). The advantages: lower wire
cost, the ability to set maximum sound levels at each speaker location, the
ability to connect a large number of speakers to a single amplifier, and a
relative lack of interaction between the volume settings in different
Low level systems are an extension of the multiple amplifier system with
one big difference: each zone has its own amplifier and that amplifier is
located at the zone (instead of system central). Advantages: lower wire
cost since the wiring to the zones is low-cost shielded cable and easier
zone programming since the zone may have its own local sources.
Disadvantages: the extra cost of multiple amplifiers (which again may be
only slightly higher than the one monster amp needed to run the whole
Now that you have an idea of what each system is, we'll get down to nuts
and bolts and describe the pros and cons in more detail.
Without a doubt, parallel distribution is the simplest system to install.
Within its limits, this system works well, doesn't require much thought on
the part of the user, and is relatively inexpensive.
The limitations are: maximum number of speakers that may be connected at
once, maximum volume level at each speaker, variations in volume when
individual speakers are turned on or off, and higher wire expense than
The impedance problem
As discussed earlier, connecting speakers in parallel reduces the
overall load impedance presented to the amplifier. Unfortunately, this
reduction in impedance means that the amplifier must work much harder to
drive the load. Connecting a load such as this directly to an amplifier is
an invitation to disaster (unless the amplifier was specifically designed
If you ever took electricity in school, then you know about series and
parallel circuits. If not, then think back to the last time that you
fiddled with a string of Christmas lights. Have you ever had a string that
wouldn't light? After searching for the bad bulb, you replace it and
voila, the entire string comes to life. This is a series string of lights.
If the circuit is broken, anywhere, the entire string goes out.
You can connect loudspeakers in series, which beneficially raises the
impedance presented to the amplifier. You can also use various
combinations of series and parallel (series-parallel). Sorry, there's no
free lunch here. The series connection makes each loudspeaker interact
with its electrical neighbors. The largest audible change is in the bass,
which may become boomy and/or tubby. If the speakers aren't the same, then
the ones with the lowest impedance will be the loudest. A series-parallel
connection may be suitable for speakers mounted in a hallway which just
need to be audible, but you only want to count them as a single speaker.
Four speakers can be connected in series-parallel and will have a total
impedance that is the same as one of them.
So you ask, "How do the commercially made switchers such as the ones made
by Adcom and AudioControl get away with this?" Easy. They insert a
resistor in series with the parallel connected remote speakers that
increases the total impedance to one that the amplifier can safely drive.
The price you pay for this is a certain portion of your amplifier power
being used to heat this resistor. This isn't quite as onerous as it may
seem. This is a very viable solution to the problem and with amplifier
power being relatively inexpensive these days, the cost of heating the
protection resistor is relatively low.
The interaction problem
Another problem faced by parallel distribution systems is that of
interaction. This means that the volume of the remote locations will
change as individual zones are switched in or out. The worst case is going
from one zone on to all zones on. With 8 ohm speakers and a typical six-
zone switcher, this can mean an overall level change of 8.5 dB which is
easily audible. If you keep a minimum of two zones on at any given time,
then the level change reduces to 6 dB which is probably tolerable.
Certainly the listeners at the remote locations can reduce the settings of
their zone volume controls.
Wire size considerations
One decided disadvantage of parallel systems is wire size. A reasonable
rule of thumb is 1 dB of loss due to wire losses. Remember that wire has
resistance, and this resistance is in the path between your amplifier and
your speaker(s). Since the wire's resistance increases with decreasing
wire size (increasing gage number), minimizing wire loss can be a
significant cost factor (larger wire means larger cost).
For a typical 50 foot run, with 8-ohm or 4-ohm speakers, here are some
Note: The wire resistance for the 50 foot run is double what it may
seem that it should be because there is actually 100 feet of wire
involved...50 going, and 50 coming.
The simplicity factor
As discussed before, parallel distribution has one strong suit:
simplicity. This is by far the simplest and easiest system to wire. If
your house is pre-wired, then anyone who can connect a pair of speakers to
a receiver should be able to install a switcher and connect it to their
receiver or amplifier.
If you like the appearance of a mountain of equipment, then this is
probably the system for you. In a nutshell, the multiple amplifier
distribution method overcomes the loading problems and impedance protection
losses of the parallel distribution method by using a separate amplifier
for each zone. Usually, this takes the form of a medium sized (30-50
Watts/channel) integrated amplifier (an integrated amplifier combines a
preamp and a power amplifier, but no tuner). Ten speakers or ten zones
means ten integrated amps.
Selecting inputs may or may not be a problem. In simple systems, it is
probably sufficient to tap from the main system at its tape output jacks,
which are unaffected by the main system's volume and tone controls. A
reasonable alternative might be a separate preamp or preamp/tuner. This
way, the distribution system in the house can receive one program while the
main system has something entirely different. By selecting the input
having the tape output from the main system, the distribution system now
mimics whatever is being played at system central.
An impedance problem of a different sort
In the parallel distribution system, the problem that had to be overcome
was that of the combined impedance of all the speakers being much lower
than most amplifiers are comfortable with. Multiple amplifier systems have
a similar problem, except that it is the load created by all the paralleled
amplifier inputs. This could create a load that most preamps are
uncomfortable with driving. The solution is simple: a distribution
amplifier, which has one input and many outputs.
Wire size considerations
This distribution method has the same limitations as the parallel
distribution method. The distribution wiring is at speaker level and
impedance and subject to the same size limitations.
Now, there's no reason why you couldn't combine a multiple amplifier system
and a parallel distribution system and have the best of both worlds. For
instance, have separate amplifiers for the master bedroom, main listening
room, and den. Now use a larger amplifier and an impedance-protected
switcher to handle the two remaining bedrooms, central hall areas, kitchen,
patio, and front entry.
How would you like to be able to connect your speakers in parallel to your
amplifier, be able to switch them on and off in any combination, be able to
preset the maximum volume level at any location, and never worry about
impedance problems (once the system planning is finished)? If your answer
was yes, then read on...a constant-voltage system may be in your future.
What is it
Basically, a constant-voltage system takes the tactic used by the power
company with the 110V wiring within your house. When you plug a lamp into
the wall, do you worry about the impedance being presented to Grand Coulee?
No way! What you do worry about is whether you'll pop the breaker because
of the 1800 watt metal-halide lamp in the greenhouse that happens to be on
the same circuit. You add wattages, and if they are less than about 2400
watts (120 volts times 20 amps), then you're golden. Constant-voltage
audio distribution systems do the same thing with loudspeakers. Add the
wattages, and if they don't exceed the rating of the power amplifier, then
you're golden again.
How does it work (ok, where's the magic??)
Constant voltage systems get their name because they operate at a constant
line voltage. In the United States, the most common voltage is 70 volts,
but 25 volts is another common voltage. What this means is that the output
voltage of the amplifier is 70 volts at full output. What this also means
is that the impedance that the amplifier will drive goes down for more
powerful amplifiers and up for less powerful ones. A couple of short
examples should help here:
The important fact here is that if you hold the voltage constant, then you
must change the impedance to change the power. This is the key to the
whole thing. Since 70 volt systems are most common, we'll use the
term "70 volt" instead of constant voltage. Remember that this
applies to all constant voltage systems, 70 volt or not. Lest you
think that this is something new, it's not. 70 volt systems have been in
common use for over 40 years!
In a 70 volt system, you have an amplifier(s) at system central, whatever
switching is needed, and a speaker equipped with a 70 volt line transformer
at each location. The switching system is just on/off switching. The line
transformers are marked in watts; you select the connections that deliver
as many watts as you wish to the speaker. Small, under 5 watt transformers
are quite inexpensive while larger ones capable of handling more power are
more expensive. Still, 10 watts is probably about right for most in-wall
speakers (if the speaker has a sensitivity of 90 dB/1 watt/1 meter, then 10
watts is 100 dB / 1 meter, or 90 dB at about 9 feet).
During the system-planning phase, you distribute your amplifier power
according to the needs of the location: 10 watts to the bedrooms(4),
kitchen, den, and living room, 2 watts each to the hallway speakers, 15
watts to the patio, 5 watts to the front walkway, and 5 watts to the entry
speakers. If you have 5 stereo pairs of hallway speakers and one stereo
pair per room elsewhere, then you're going to need a 105 watt amplifier to
make things run. Only got 100 watts? Simple...reduce the patio to 10
watts, or reduce the front walkway and entry speakers to 2 watts each.
Remember, it doesn't matter if you come out under your amplifier's rated
- A 30 watt amplifier delivers 70 volts into its load at full output.
What is the impedance of that load? Using Ohms law, R = E2 / P.
70V X 70V / 30W = 163 ohms. Stated another way, a 163 ohm resistor
with 70 volts across it dissipates 30 watts.
- A 612 watt amplifier delivers 70 volts into its load at full
output. What is the impedance of the load?
R = E2 / P. 70V X 70V / 612W = 8 ohms.
Wire size considerations
In a 70 volt system, the speaker impedance is magnified by the line
transformer (the real secret to 70 volt systems is calculated, pre-
meditated impedance mis-matching!). Thus, it takes much more wire
resistance to cause 1 dB line loss. Translation: you can use smaller
Zone level tailoring
If you guessed from the lead-in to this section, one of the beauties of
this method is the ability to preset volume levels at each speaker
location. Furthermore, if you want a local volume control, install one,
but after the 70 volt line transformer. Even if you use an L-pad, the
worst that will happen is the L-pad soaking up as many watts as the line
transformer is connected for.
Not all amplifiers are equipped for 70 volt operation. In fact, most
consumer-grade amplifiers aren't. Aside from buying a commercial/rock-n-
roll grade amplifier that has an integral 70 volt output, you can do either
of the following and use a consumer-type amplifier:
- Use a step-up transformer connected to the amplifier's output to
increase its output voltage to 70 volts at full output. Many
manufacturers make these available for their amplifiers (although not
too many consumer manufacturers do). You must get a model with
sufficient power capability to handle the full output of your amplifier
as well as the proper turns ratio to ensure the proper voltage step up.
- Operate the amplifier in mono-bridge mode. If the amplifier can
deliver about 612 watts to an eight-ohm load, then its output voltage
is around 70 volts. No transformer is needed at the amplifier end
(although you still need the transformers at the remote speaker end).
A plea for sonic sanity
You may have noticed a certain 11-letter word throughout the preceding
paragraphs: transformer. You know, 2 coils of wire wound together on an
iron core? Transformers have been blamed for many audio ails and some of
them are deserved. Yes, you get what you pay for, and generally speaking,
cheap transformers are that way because the manufacturer skimped on the
core material (iron), and that means substandard bass response. Any dyed-
in-the-wool purists may be retching by now and that's fine...for them.
I'm not advocating that you put the Martin-Logan's in your main listening
room into a 70 volt distribution system. What I am advocating is a bit of
objectivity and practicality in putting the less critical parts of your
system into a 70 volt system. Again, remember that you don't have to put
the ENTIRE house into the system. Just remember that for some locations
(hallways, entry-ways, walkways, bathrooms, etc.) a 70 volt system may be
just what the doctor ordered since you don't need enormous amounts of power
at any time, and the system isn't likely to be critically listened to.
This plea for sonic sanity also applies to wire. If you believe in
pedigreed wire, fine...save it for the Martin-Logan's. If you want to use
it in the walls for the 70 volt system, fine...it's your money...but plain-
vanilla twisted-pair stranded wire will probably work every bit as well.
Are you really going to hear the improvement in soundstage when you walk
down the hall?
Low Level Distribution
Low level distribution is really just a twist on the multiple amplifier
method. Instead of making a mountain of amplifiers at system central, you
put an amplifier into each room/zone. The local amplifier doesn't need to
be huge...30 to 50 watts/channel should be more than adequate. If you buy
small receivers instead of integrated amplifiers, then the local user has
the option of a tuner (set to their favorite station) or whatever is on the
main system. The wiring to each room is just shielded cable, usually
shielded twisted pair, and each location just taps off of the cable.
Really elaborate systems may have several cables run through the house so
that the remote locations have the pick of several programs. With 6 cables
run, each location could have the pick of two CD changers (one with
classical music, the other with pop), or whatever is running on the main
What are the pitfalls?
Not many, really. The biggest one is driving the cable. You need a
distribution amplifier to do this. The tape output of your preamp won't do
this well at all, and you really need isolation between the main system and
the distribution system.
Another pitfall is ground loops. If you're careful, you can run the
distribution system unbalanced and get away with it. At a minimum, its a
good idea to have a transformer coupling into the distribution line. This
way the grounding at the remote locations is much less likely to affect the
main system. If the grounding becomes a problem, then you'll probably have
to treat the distribution line as a balanced line and perform a balanced-
to-unbalanced conversion at each remote location.
Volume Controls and Switchers
No discussion of remote audio distribution systems would be complete
without a discussion of volume adjusting devices.
An L-pad is nothing more than a special type of adjustable resistor. It
differs from the common potentiometer because it presents a constant,
unchanging load to its source in spite of its setting. A potentiometer
presents a changing load to its source whose value depends on the shaft
position. L-pads are usually found in loudspeaker crossover networks as
balance controls (like the tweeter level control in a 2-way system). Since
they present a constant impedance to the crossover regardless of setting,
the crossover's characteristics are less likely to be altered by the
setting of the control.
Nevertheless, both devices convert the unused portion of the audio signal
into heat. In a simple parallel distribution system, an L-pad is a poor
choice for a zone volume control because it must be able to withstand the
amplifier's full output power. If the amplifier was rated at 100 watts,
then you'd need a 100 watt L-pad! Such units are difficult to find, and
expensive. In a 70 volt system, the L-pad is a good choice for a zone
volume control because it only dissipates the wattage assigned to its zone.
It also has the advantage of being almost infinitely variable (no click-
An auto transformer is a variation on the more common two winding
transformer. In this case, both the primary and secondary are part of the
same winding. Auto transformers are popular as zone volume controls
because they don't convert the unused audio signal into heat. When used as
a volume control, intermediate taps on the winding are brought out to a
rotary switch, which selects one of the taps for connection to the
loudspeaker. The closer the tap point is to the input terminal, the lower
the loss through the auto transformer (translate: louder).
Auto transformers suffer from many of the same problems as transformers.
Not enough iron results in poor low-frequency response, or distortion at
high levels. On the other hand, actual units measured in a lab setting
revealed a lesser dependence on physical size (iron content) for reasonable
performance. I wouldn't hesitate using one of these units as a zone volume
Switchers and Impedance Protection schemes
A seemingly simple part of most in-home systems is a speaker switcher.
From the outside, it doesn't seem like anything could be much simpler. If
only it was...
For speaker level distribution systems, the primary difficulty is one of
impedance. When all speakers are switched on, the resulting load is too
great (impedance value too low) for most amplifiers to drive. Trying to do
so to an amplifier that wasn't specifically designed for such loads results
in distortion, overheating, and/or smoke.
Several manufacturers make simple switchers that incorporate an "impedance
protection" feature. In most cases, this takes the form of a large power
resistor. The resistor connects in series with the speakers and prevents
the total impedance from falling below what is safe for most amplifiers.
The advantage of this scheme is utter simplicity. The disadvantage is
power loss. This is discussed more fully in the previous section on the
parallel distribution method.
If your system is a 70 volt, or constant-voltage system, then you can use a
simple switcher with no impedance protection resistor. This is one of the
big advantages of constant-voltage distribution.
As you can see, there are more than a few ways to wire your home for sound.
To sum things up, the following chart relates all of the systems described
to each other. In the chart, each plus counts as one point, each minus
counts as -1 point. A superlative in a category counts as 2 points (i.e.
lowest, cheapest, best ...). The "score" is the sum of all the pluses and
As you can see, there is no clear-cut winner. Every system has its
strengths and weaknesses. Every system has something to say for itself.
Your choice of a system should be driven by cost, needs, and simplicity
considerations. For most systems, especially those that you install
yourself, the parallel system using an impedance protected switcher is
probably the winner. Larger systems, especially those with many zones are
good candidates for a constant voltage system. Cost-no-object systems are
probably best served by low level systems with locally located amplifiers.
# of speakers
|para. /w switcher
# of speakers
reduced damping factor
wire size needed
||moutain of gear
different progams in each zone possible
|mountain of gear
wire size needed
distribution amp needed
low wire cost
|probably highest cost
possibility of ground loop trouble
zone level tailoring
of amp power
lower wire cost
marginally higher complexity